A conventional laser radar system detects for information, such as information pertaining to time and distance, and evaluates how far a distant object is by time-of-flight (TOF). An advanced conventional laser radar system estimates characteristics of effective reflectance of the surface of a distant object according to the strength of a signal received at a receiving end. A conventional multi-wavelength system further provides relatively limited information pertaining to the spectrum of a distant object; however, the conventional multi-wavelength system gathers limited information required for identifying the distant object, because it cannot directly estimate coarseness and tilt angle of the surface of the object or judge whether the object is artificial, unless by higher spatial resolution, such as joint use of 3D imaging and an intricate algorithm, whereby it can judge by appearance as to whether the object is a vehicle, pedestrian or the like. However, the aforesaid technique not only entails gathering much data but also requires robust computation capability in order to effectuate automated judgment. As a result, the aforesaid judgment can hardly be achieved by a laser radar system with low to medium resolution.
Take a laser radar or laser range finder as an example, the strength of an optical signal provides information pertaining to effective reflectance of an object which a laser beam is reflected off, wherein the effective reflectance depends on the object's surface characteristics, such as coarseness, scattering, and the included angle between the object's surface and incident light. Furthermore, polarization information provides an opportunity to evaluate physical characteristics of the object's surface and even determine whether the object is made of a natural substance or an artificial substance. FIG. 1 shows the contrast between a nearby coarse surface and a highly smooth surface obtained by polarization information.
In general, the light for use in measurement polarization information is usually randomly polarized, and the polarization detection must vary at a detection end, and in consequence a component with a variable polarization direction must be mounted at the front end of a detector as far as flash imaging is concerned. However, the aforesaid technique is likely to mix ambient light and signal light. Furthermore, if a laser radar system uses multiple detectors, a component with a variable polarization direction must be mounted at the front end of each detector, thereby incurring high costs and increasing system complexity.